Permanent magnets of the type used in electric motors, linear actuators and the like are created by magnetizing magnetically hard materials. This type of magnetization is usually accomplished by placing the material within a high intensity magnetic field which is created by passing a very high electric current pulse through a coil or coil-pole structure usually called a magnetizing fixture. The current pulse is created by the sudden release of charge stored in an energy storage device such as a bank of capacitors. The high current pulse resulting from the sudden release of the charge passes through the windings of the magnetic fixture and causes a brief but very strong magnetic field which, in turn, causes the magnetic domains within the magnetically permeable material to align in the required pattern. This alignment is achieved within the material in a very brief period of time (less than a few millionths of a second). Once the magnetic field of the fixture collapses, the material remains permanently magnetized.
Magnetizing fixtures typically comprise coils of an electrical conductor whose windings are separated from one another and from other parts of the fixture by an insulator. The insulator may comprise solid insulating material, such as a dielectric lacquer coating on the conductor, or simply the air separating the windings and other parts. In any case, the insulator has a breakdown voltage which, if exceeded, will at least prevent proper operation of the magnetizing fixture and usually destroy or damage solid insulation. This limits the rate at which the current in the conductor may change, the voltage on the conductor being proportional to the rate of change of current through the conductor.
When current passes through a conductor, the resistance of the conductor results in an energy loss, which produces heat. That energy loss, and concomitant heat, is proportional to the integral of the square of the current through the conductor with respect to time. Not only does the energy that is converted to heat represent energy that cannot be used for magnetization, but the heat produced by this power loss can damage the fixture, particularly any solid insulation that is employed therein. Therefore, to reduce heating of the fixture, it is desirable to keep the current pulse through the conductor of the fixture as short as possible.
It can be shown that, to meet the conflicting goals of keeping the voltage on the conductor of the magnetizing fixture below the insulation breakdown voltage and keeping the magnetization current pulse as short as possible, the current produced by the magnetizing pulse should increase and decrease linearly with time. It can also be shown that it is desirable to hold the maximum current for a brief period of time to overcome eddy currents, if present. Since the voltage across the magnetizing fixture is proportional to Ldi/dtxe2x88x92iR, where L is the inductance of the fixture and R is the resistance of the fixture, the ideal magnetization current pulse would be one that has a trapezoidal shape, the increase in current to its maximum and the decrease in current to its minimum being linear so as to produce a constant voltage on the conductor that is less than the breakdown voltage.
Known prior art methods and systems have not been able to achieve a substantial approximation of the ideal magnetization current waveform. While there are circuits available for producing a signal having a triangular shaped waveform, and perhaps even a trapezoidal shaped waveform, none is known which is able to do so at the current levels necessary for driving a magnetizing fixture, that is, hundreds to thousands of amperes. Those known circuits for driving a magnetizing fixture produce magnetization waveforms whose rate of change of current changes significantly over the time of the magnetization pulse so as to produce voltage peaks that must be kept below the breakdown voltage of the insulation of the fixture. The effect of having to keep the voltage peaks below the breakdown voltage is to limit the amount of magnetization current that can be supplied to the magnetizing fixture, and thereby limit the amount of magnetization that can be produced in a permanent magnet. This is because, to avoid overheating, the magnetization current pulse cannot be applied long enough to reach the maximum possible current.
Therefore, there has been a need for a method and system for driving a magnetizing fixture so as to produce a substantially triangular magnetization current waveform reaching a peak current in excess of 100 amperes.
The present invention meets the aforementioned need by providing a method and system that produces in the conductor of a magnetizing fixture for a first period of time a first current of whose magnitude increases substantially linearly with time to a peak value of at least 100 amperes, and thereafter produces in the conductor of the magnetizing fixture for a second period of time a second current in the same direction as said first current whose magnitude decreases substantially linearly with time. The sum of the first and second periods of time are typically less than about 0.1 seconds. The invention may provide in addition the production in the conductor of the magnetizing fixture of a third current in the same direction as first current whose magnitude is within a predetermined range of said magnitude of said first current at the end of said first period of time and before the second period of time, so as to provide a brief dwell time between the first and second periods of time to allow eddy currents in the fixture to die out. These three currents together form a substantially triangular magnetization current waveform that is trapezoidal having substantially symmetrical sides.
One circuit for driving a magnetizing fixture for producing the aforedescribed currents comprises a first energy storage device for supplying electrical energy to,the magnetizing fixture, a second energy storage device for receiving electrical energy from the magnetizing fixture, and a commutator interconnecting the first energy storage device, the second energy storage device and the magnetizing fixture for stopping the flow of electrical energy into the magnetizing fixture from the first energy storage device and starting the flow of energy from the magnetizing fixture into the second energy storage device. A third energy storage device is provided for supplying electrical energy to the magnetizing fixture after the first energy storage device but before the second energy storage device to commutate the first energy storage device off and supply current to the fixture while decreasing voltage across the magnetizing fixture to the point at which the second energy storage device begins to receive energy from the fixture.
The rate of change of current during the first and second periods of time is controlled to be the maximum possible without exceeding the breakdown voltage of insulation in the magnetizing fixture.
Accordingly, it is a principal object of this invention to provide a method and system for driving a magnetizing fixture wherein the maximum magnetic field can be produced with minimum generation of heat in the magnetizing fixture.
It is another object of this invention to provide a current pulse in a magnetizing fixture whose waveform is substantially triangular and reaches a peak current in excess of 100 amperes.
It is a further object of the invention to provide such a current pulse having a duration less than about 0.1 seconds.